Re: [CAD_CAM_EDM_DRO] Re: Plasma Voltage Servo

> My question
> is 1. will step and direction signals from 2 different sources
> (Mariss's Circuit and the computer)Conflict with each other if they
> are wired to the same drive even if they arent giving commands at the
> same time? And 2. If they do conflict, could a Double pole double
> throw relay at the drive input set up so as to toggle between the
> two command sources when ordered to toggle by a spindle moter
> command. I probably wouldnt tie it to the plasma on off command
> because I would like the torch to lower before the plasma turns on
> and raise after it turns off. Again the air system is a nice Idea,
> But I'm trying to reduce the need for extra mechanical assemblies if
> I can accomplish it all with the same Z axis.

Dale,

Since I haven't seen an answer to your question (or missed it if there
was one) I'll take a stab at it.

There are ways to combine signals from two sources, but you would have
to think out carefully how you want to combine them.

The torch height control is easy to deal with since it is based on
feedback - as long as the torch is too low, it will be generating up
commands, and the reverse. So it can probably deal with being handed
control in any position the computer leaves the torch at.

But the computer height control is trickier - since the computer has
no way of knowing where the arc control has left the torch head. One
method would be to add an encoder so the computer knows. Another
would be to make assumptions about where the height control leaves the
z axis. A nifty approach might be to use the computer to change the
desired voltage which the height control is trying to achieve
(possibly even to an unachieveable value, if experiment shows this
will drive the head to the plus or minus limit) and control the height
that way.

Whatever you do, you will probably need switches and logic gates to
implement smart limits - ie, the max height limit should prohibit
further movement up, but allow movement back down down. OTOH, if your
stepper doesn't need to be too strong you can probably just turn down
the current and let it stall against physical stops without hurting
anything.

Mode switching is a possibility, but you should not use a relay for
this. Relay contacts, like switch contacts, can 'bounce' when they
are actuated - that is, they can connect and disconnet a few times
before settling out to the desired state. This happens faster than
you can see, but electronics are fast enough to possibly misinterpret
this bouncing as step pulses. So you should use integrated circuit
multiplexors or logic gates to do mode switching.

Actually, you might be able to tolerate some bounce-induced false
steps since the torch height control will work to recover from any
height error they induce.

What you really need to do is sit down with a sheet of paper and
devise the 'rules' for controlling the z axis - when should it be in
arc voltage mode, when should the computer move it, and which control
has priority when they overlap. Once that is figured out, a circuit
to implement these rules is relatively simple. One of the things you
will want to do in preparation for this is to build Mariss's height
control circuit and see what it does when you turn off the torch, if
you turn it on a long ways from the work, and possibly what happens if
you set the desired voltage level unreasonably high or too low - ie,
could the computer simply control that when it needs to move the head.

Chris Stratton

-------------------------------------------------
Christopher C. Stratton
Engineer, Instrument Maker, and Horn Player
ccs@... 617 628 1062
http://web.mit.edu/~stratton/www/brassbuild.html